SPECIFIC GRAVITY, a comparison of every solid, or fluid, with the weight of the same magnitude of rain-water. Rain-water is chosen as the standard of comparison, on account of its being less subject to variation in different circumstances of time, place, &c., than any other body, whether solid or fluid. And by a very fortunate coincidence, at least to English philosophers, it happens, that a cubic foot of rain-water weighs 1,000 ounces avoirdupoise ; and con sequently, assuming this as the specific gravity of rain-water, and comparing all other bodies with this, the same numbers that express the specific gravity of bodies, will at the same time denote the weight of a cubic foot of each in avoirdupoise ounces, which is a great convenience in numerical com putations.
From the preceding definition, we readily draw the follow laws of the specific gravity of bodies, viz., 1. In bodies of equal magnitudes, the specific gravities are directly as the weights, or as their densities.
2. In bodies of the same specific gravities, the weights will be as the magnitudes.
3. In bodies of equal weights, the specific gravities are inversely as the magnitudes.
4. The weights of different bodies arc to each other in the compound ratio of their magnitudes and specific gravities.
it is obvious, that if in the magnitude, weight, and specific gravity of a body, any two be given, the third may be found; and we may thus find the magnitude of bodies, which are too irregular to admit of the application of the common rules of mensuration ; or we may, by know ing the specific gravity and magnitude, find the weight of bodies which are too ponderous to be submitted to the action of the balance or steelyard ; or, lastly, the magnitude and weight being given, we may ascertain their specific gravities.
Example 1.--The weight of a marble statue being 7481b. avoirdupoise, required the number of cubic fret, &c., which it contains; the specific gravity of marble being 2742.
Since a cubic foot weighs 2742 ounces, we have as 2742 : 748 X 16 : : 1 : 4.36 feet.
Example the weight of a block of omit( whose length is 63 feet, and breadth and thickness each 1f.'. feet ; the specific gravity of granite being 3500.
here 63 x 12 x 12 = 9072 feet: then again as 1 : 9072 : : 3500: 31752000 ounces : or 885 tons, 181- cwt. The above are said to be the dimen. sions of one of the stones in the walls of Balbec.
Other properties relating to the specific gravity of bodies are as follows, viz.
1. A body immersed in a fluid will sink, if its specific gravity be greater than that of the fluid; it be less, the body will rise to the top, and be only party immersed ; and if the specific gravity of the solid and fluid be equal, it will remain at rest in any part of the fluid in which it may be placed.
2. When a body is heavier than a fluid, it loses as mud of its weight when immersed, as is equal to a quantity of the fluid of the same bulk or magnitude.
3. If the specific gravity of the fluid be greater than that of the body, then the quantity of the fluid displaced by the part immersed, is equal to the weight of the whole body, And hence, as the specific gravity of the fluid is to that of the body, so is the whole magnitude of the body to the part 4. The specific gravities of equal solids are as their parts immersed in the same fluid.
5. The specific gravities of fluids are as the weights lose by the same immersed solid.
Hence are drawn the following rules for ascertaining th( specific gravities of both solids and fluids.
To find the specific gravity of a may be don( generally by means of the hydrostatic balance, which is con trived for the easy and exact determination of the weight: of bodies, either in air, or when immersed in water, or other fluid, from the difference of which the specific gravity of botl the solid and fluid may be computed.
1. When the body is heavier than it boa out of water and in water; then say, As the weight lost in water Is to the whole or absolute weight, So is the specific gravity of water To that of the body.
2. When the body is lighter than this case attach to it a piece of another body heavier than water, se that the mass compounded of the two may sink together Weigh the denser body and the compound body separately both out of the water and in it; and find how much cad loses in the water by subtracting its weight in water from it: weight in air; and subtract the less of these remainders from the greater. Then use the following proportion : As the last remainder Is to the weight of the light body in air, So is the specific gravity of water To the specific gravity of the body.
3. When the specific gravity of the fluid is required— Take a piece of some body of known specific gravity; weigh it both in and out of the fluid, and find the loss of weight by taking the difference of these two; then say, As the whole or absolute weight Is to the loss of weight, So is the specific gravity of the solid To the specific gravity of the fluid.
The following table exhibits the specific gravity of several of the most common bodies ; it is extracted from a more extensive one given in Gregory's Mechanics; and other tables of a similar kind will be found in the works of Emer son, Muschenbrocck, Ward, Cotes, Martin, (Sm.